The micromere-primary mesenchyme cell lineage of the sea urchin is a model system for investigating cellular and molecular aspects of cell differentiation. This lineage gives rise to the calcareous skeleton (spicule) of the embryo. Spicule formation requires intimate association between the primary mesenchyme and the extracellular matrix (E of the embryo. We have developed a model system utilizing an exogenous ECM or serum which promotes mesenchyme differentiation in vitro. Our first aim is to isolate, characterize and raise antibodies to the spicule inducing activity (SIA) in the ECM. Our second aim is to identify transcripts in the micromeres which are induced by the serum or extracellular matrix. These transcripts presumably represent early gene products necessary for overt differentiation of this lineage. The second aim is independent of the first in that we need not identify the SIA in order to isolate the induced genes and their mRNAs.